Method and apparatus for measuring body fat

ABSTRACT

A body fat measuring apparatus for measuring body fat in a patient&#39;s body comprises apparatus for simultaneously measuring an impedance between the patient&#39;s feet, and for measuring the patient&#39;s height and the patient&#39;s weight, and a calculator for calculating body fat from the measured impedance value, height and weight. The calculator estimates body density using a formula having a correction term either for increasing the body density upon increase of the impedance compared to the original impedance or a correction term for decreasing the body density upon increase of the weight with respect to the original weight.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of patent application Ser. No.08/224,087, filed Apr. 6, 1994, now U.S. Pat. No. 5,415,176, which is acontinuation-in-part of patent application Ser. No. 07/948,606, filedSep. 23, 1992, now abandoned.

BACKGROUND TO THE INVENTION

1. Field of the Invention

This invention relates to a method and apparatus for measuring body fatfor measuring the amount of fatty tissue in vivo.

2. Description of the Prior Art

It has been customary to determine whether or not a human being is fatfrom the relationship between body weight and height.

However, it is not always unhealthy that weight is large in comparisonwith height, and it is important to measure fatty tissue in vivo. It ispractical to measure body fat in vivo by measuring body weight.Recently, various body fat measuring instruments have been sold in themarket.

An underwear body weighing method is known for estimating body fat bycalculating body density from the measured weight value under water as amethod for accurately measuring body fat in vivo.

This underwater body weighing method needs a large facility, andnecessitates skillful measuring techniques and an effort by the personbeing measured considering the large influence of the amount of residualair in the lungs to be measured.

In order to eliminate the disadvantages of the above-describedunderwater body weighing method, there has been proposed a method forestimating body density by measuring body impedance between extremitiesof the body and estimating the body density from the measured impedancevalue, height and body weight.

The ratio of body water in fat-free tissue of body composition tissue isconstant and the specific resistance of the fat-free tissue is constant.The estimation formula: bodydensity=1.1554−0.0841*weight*impedance/(height)² of Segal et al., andbody formula: density=1.1303−0.726*weight*impedance/(height)² of Naka etal., by obtaining the inference formula as represented by bodydensity=A−k*weight*impedance/(height)², and obtaining correlationbetween the constant A and the value obtained actually by an underwaterbody weighing method for determining the constants A and k, aredisclosed.

However, it is known that the impedance value varies due to variousfactors such as variation by body water ratio in fat-free tissue due tovariation in body weight, amount of blood plasma due to variation inmotion or attitude and movement of interstitial liquid. In order toaccurately measure the impedance value, it is necessary to considerinfluences of ingestion of food and water, motion, attitude andvariation in body weight.

Thus, the impedance value of a body to be measured vary according to thedate, time and differences of body condition before measurement. Thesevarious are too large to use the above-described inference formulaaccurately and amount of body fat calculated by the formula is likely tobe inaccurate.

Therefore, in order to measure accurately, it is necessary to maintainthe body state of the patient to be measured constant by selecting a dayfor taking the measurements when ingestion of food and water, andactivity, are limited and variation in body weight is small, and tomeasure the impedance value after the patient has been lying on a bedfor a predetermined period of time. Thus, the measuring conditions areknown.

On the other hand, there has also been proposed a method for calculatingbody fat from numeric values with respect to a body such as an impedancebetween extremities of the body, height, weight and gender by measuringthe impedance using a 4-terminal Kelvin bridge living body impedancemeter as a method for accurately and simply measuring the body fat invivo as in U.S. Pat. Nos. 4,008,712 and 4,895,163 (Japanese PatentLaid-open No. Hei. 2-60626).

Since the degrees of bending and the angle of the limbs influence theimpedance values using these methods, the measuring conditions when thepatient is lying on a bed are maintained constant to eliminate errors.

Further, if the spacing between a current supply terminal and a voltagedetecting terminal of electrodes for detecting the impedance is notsufficient, the measured values are effected, and hence the spacing ismaintained by mounting the current supply terminals on the backs of thehand and foot and mounting the voltage detecting terminals on thetendons of the wrist and the ankle.

Accordingly, restrictions in the measuring position are significant(needs a bed, etc.), and it is difficult for a person to use theapparatus to take measurements of himself without assistance.

Moreover, in a method for measuring living body impedance, it has alsobeen necessary to measure body fat as described above, using a methodhaving steps of outputting a sine wave of 50 kHz from an oscillator as aconstant-voltage source, converting it to a constant current of 800microamperes by a voltage/current converter, supplying the currentthrough a pair of electrodes mounted on a body, outputting the voltagevalue of a voltage drop from a pair of electrodes mounted inside theabove-described electrodes by a differential amplifier,waveform-shaping, rectifying it, DC-converting it, then, A/D-convertingit as digital data, and applying it to a calculator, determining theliving body impedance Z by Z=V/I from the living body current I and theterminal voltage V, and measuring the voltage V when the current I isConstant, thereby determining the living body impedance Z.

However, the constant-current source is affected and its output isvaried due to the influence of the external environmental temperature,etc., by the degree of variation in the electrode impedance, and it isdifficult to obtain an accurate value of the living body current I as anaccurate constant current, thereby causing error in measuring the livingbody impedance.

On the other hand, the living body impedance to be measured by a bodyfat meter needs to be accurate in a wide range of 0 to about 1 kilo-ohm.It is extremely difficult to obtain an ideal proportional relation inthis range, thereby necessitating a great deal of work for makingcorrections to the measured value.

Accordingly, an object of this invention is to provide a method andapparatus for measuring body fat which has a small restriction inmeasuring conditions and which can be simply measured without influenceby the measuring conditions.

Another object of this invention is to provide a practical body fatmeasuring apparatus which can be readily used by the person beingmeasured without restriction.

Still another object of this invention is to provide a living bodyimpedance measuring method which is particularly adapted for measuringbody fat and can accurately measure the absolute value of the livingimpedance in a wide measuring range without influence of theenvironment.

SUMMARY OF THE INVENTION

According to an aspect of this invention, there is provided a method formeasuring body fat by the steps of measuring impedance betweenextremities of a body, estimating body density from the measuredimpedance value, measuring the height and the weight, and calculatingthe body fat, comprising the step of estimating a body density by acalculation formula having one of a correction term for increasing thebody density upon increase of the impedance with respect to only theimpedance and a correction term for decreasing the body density Uponincrease of the weight with respect to only the weight.

According to another aspect of this invention, there is provided a bodyfat measuring apparatus for measuring body fat in a patient's bodycomprising a measuring station for simultaneously measuring an impedancebetween extremities of the patient's body, a patient's height and apatient's weight, and a calculator for calculating body fat from themeasured impedance value, height and weight, whereby the body density isestimated by a calculation formula having one of a correction term forincreasing the body density upon increase of the impedance with respectto only the impedance and a correction term for decreasing the bodydensity upon increase of the weight with respect to only the weight.

According to still another aspect of this invention, there is provided abody fat measuring apparatus having the steps of measuring an impedancebetween extremities of a body, and calculating body fat from numericvalues of physical conditions such as the measured body impedance value,height, weight, and gender comprising a pair of conductive electrodesfor contacting the patient's toes defined as current supply terminals ata nearest interval of 5 cm or more, and a pair of electrodes forcontacting a patient's heels defined as voltage detecting terminals onan insulating base having protrusions for heel guides in such a mannerthat, when the patient stands thereon, the electrodes are mounted toprotrude from the base symmetrically with respect to the width of thebase spaced apart for the patient to stand in an upright position sothat the patient's feet are not touching and the patient's bodyimpedance can be measured in the standing position.

According to still another aspect of this invention, a method formeasuring impedance of a person is provided which comprises the steps ofinserting a plurality of reference resistors in a current path forsupplying a current to a living body, measuring voltage drops of thereference resistors by momentarily switching the reference resistorgroup and the living body to obtain a correlation formula between themeasured value by the reference resistors and the impedance, anddetermining the impedance of the living body by the patient's measuredvalue and the correlation formula.

This invention will not be described in further detail with respect topreferred embodiments as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a body fat measuring apparatusshowing an embodiment of the present invention;

FIG. 2 is a plan view showing, in detail, a base of a body weighingapparatus of FIG. 1;

FIG. 3 is a sectional view taken along the line 3—3 of FIG. 2; and

FIG. 4 is a block diagram showing an example of a circuit arrangementfor carrying out a living body impedance measuring method according tothe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a body fat measuring apparatus is shown as anembodiment of the present invention. This apparatus comprises anelectronic body weighing meter 5 having a base 1 on its upper surface,an electronic height meter 4, and a display unit 6. This electronic bodyweighing meter 5 electronically calculates a patient's body weight whena patient stands on the base 1. The electronic height meter 4 detectsthe height of a patient standing on the base 1 using a detector 4A andelectronically calculates the height value.

As shown in FIGS. 2 and 3, the base 1 formed of an insulating material,and a pair of conductive electrodes 3 for a patient's heels andconductive electrodes 2 for a patient's toes are mounted symmetricallywith respect to the width of the base spaced to allow the patient tostand upright so that the patient's feet are not touching when thepatient stands thereon on both feet. The electrodes 2 and 3 are set to 5cm or more apart so as to eliminate the influence of a potentialdistribution. The electrodes 2 are formed in an elliptical shape so asto correspond to various sizes of a patient's foot. The electrodes 2 areused as current supply electrodes, and the electrodes 3 are used asvoltage detecting terminals.

Heel guides 7 are provided at the rear of the electrodes 3 so that auser can always stand at a predetermined position on the base 1 with hisheels in contact with the electrodes.

An electronic circuit for carrying out a body impedance measuring methodaccording to the present invention is placed in the housing of theelectronic body weighing meter 5, and an example of the electroniccircuit as shown in a block diagram of FIG. 4. In FIG. 4, electrodes Acorrespond to the electrodes 2 for the patient's toes in FIGS. 1 to 3,and electrodes B correspond to the electrodes 3 for the patient's heelsin FIGS. 1 to 3.

The body impedance measuring electronic circuit in FIG. 4 measures apatient's body impedance when a patient stands on the base 1 of theapparatus in FIG. 1 in such a manner that the patient's toes and heelsare contacted correspondingly with the electrodes 2(A) and theelectrodes 3(B).

An oscillator 11 generates a sine wave of 50 kHz as a constant-voltagesource, which is converted to a constant current of 800 microamperes bya voltage/current converter 12, which does not affect influence tolinearity of a body impedance. The constant current is supplied from apair of electrodes A through a plurality of reference resistors 12, 14,15 having known resistances to cover an average measuring range (a rangenecessary to measure an impedance between the extremities of a patient'sbody as individual differences: 0 to 1 kilo-ohm).

Values of voltage drops of the reference resistors 13, 14, 15 in agroup, and values of voltage drops of the pair of electrodes B mountedinside a living body from the electrodes A are momentarily switched by aswitch 16 to be controlled by a microprocessor unit (MPU) 21 whichincludes a calculator, and differences therebetween are output by adifferential amplifier 17. The voltage value output by the differentialamplifier 17 is rectified by a rectifier 18, waveform-shaped by a lowpass filter (LPF) 19 to be DC-converted, A/D-converted by ananalog/digital converter (AD) 20, and input as digital data to bemicroprocessor unit 21 obtains a correlation formula between themeasured value of the reference resistor and the impedance, anddetermines a living body impedance from the measured value of thepatient's living body and the correlation formula.

An electronic circuit for using in a body fat measuring method accordingto the present invention is also associated in the housing of theelectronic body weighing meter 5. This body fat measuring electroniccircuit calculates a patient's body fat as below from the patient'sweight measured by the electronic body weighing meter 5, the patient'sheight measured by the electronic height meter 4 and the patient's bodyimpedance calculated by the above-described body impedance measuringelectronic circuit.

An inference formula of body density (DB) of Segal et al., using weight(W), height (Ht) and impedance (Z), is represented by:

DB=A−k*W*Z/Ht²  (1)

where

A is a constant,

k is a proportional coefficient

The value proportional to the weight and the impedance Z and inverselyproportional to the square of the height is subtracted from the constantA.

However, as described above, it is known that the impedance value variesdue to various factors such as variation in body water ratio in fat-freetissue due to variation in weight, blood plasma due to variation inmotion or attitude and movement of interstitial liquid in a shortperiod. The variation in the impedances greatly influences the bodydensity in the formula (1), but the variation factor of the impedancedoes not originally greatly vary the body density.

In order to reduce the influence of the variation in the impedances tothe body density, a correction term for reversely affecting theinfluence of the variation in the impedances of the formula (1) of thebody density, i.e., a correction term for increasing the body density ifthe impedance is increased is provided as a third term, and a correctedreference formula is represented by:

DB=A−k′*W*Z/Ht²+k″*Z  (2)

where k′ and k″ are proportional coefficients.

On the other hand, the above-described assumption is based on thepremise that the variation in the weight is small with respect to thevariation in the impedance, and it can be corrected if the variation inweight is responsive to the variation on the measured impedance. It isreplaced by providing the correction term for performing the sameoperation as the influence of the variation in the weight in the formula(1) to the body density, i.e., the correction term for reducing the bodydensity if the weight is increased as a third term, and a correctionformula is represented by:

DB=A−k′*W*Z/HT2−k″*W  (3)

In order to reduce measuring conditions of an impedance between apatient's feet, with the patient in a standing attitude, and toaccurately measure the impedance, tests were conducted by a number ofsubjects under various conditions to obtain correlation between theabove-described assumption and an underwater body weighing method, andmost desirable coefficients in the formulae (2) and (3) have beendecided.

The coefficients A, k′ k″ may be determined experimentally and/orstatistically so that the equations provide measures of body densitywhich most closely approximate those measured by means of an underwaterbody weighing method.

The following formula could be obtained as a most suitable inferenceformula for estimating a patient's body density by using the impedancevalue between both of the patient's feet in a standing position based onthe formula (2):

Body density=1.1144−0.0976*W*Z/Ht²+0.000084*Z

where

W: body weight (kg)

Z: impedance (ohm)

Ht: height (cm)

On the other hand, the following formula could be obtained as a mostsuitable inference formula for estimating a patient's body density byusing the impedance value between both of the patient's feet in astanding position based on the formula (3):

Body density=1.1442−0.0626*W*Z/Ht²−0.00044*W

where

W: body weight (kg)

Z: impedance (ohm)

Ht: height (cm)

Body fat can be obtained by the following formula:

body fat (%)=(4.57/body density−4.142)*100

The correlation to the underwater body weighing method of the result forcalculating the body fat by the above-described inference formula is ofthe same degree as that for fixing the measuring conditions without acorrection term, the variation due to the difference of the measuringconditions is reduced to ½ of that without the correction term, therebyobtaining a satisfactory result.

According to the present invention, body fat can be concurrentlymeasured in a standing position with extremely small error in themeasurements by correcting the inaccuracy of the measured values due tothe variation in the impedance value according to the measuringconditions.

Further, the body fat measuring apparatus according to the presentinvention can readily be used by a user to measure his body fat byhimself merely be standing upright at a designated position, and he canrepeatedly measure it under the same conditions and can also take themeasurements in the standing position. Therefore, restrictions in themounting position of the measuring instrument are remarkably reduced.

We claim:
 1. A method of measuring impedance of a patient's bodycomprising the steps of: connecting a plurality of reference resistorseach having a known resistance in series with said patient's body;supplying an electric current into a current path through said pluralityof reference resistors and said patient's body; measuring voltage dropsacross any of said reference resistors and/or measuring voltage dropsacross any groups of adjacent said reference resistors, and measuring avoltage drop due to said patient's body by momentarily and successivelyswitching over a voltage drop measuring device for each of said voltagedrops; obtaining a correlation formula between said known resistances ofsaid reference resistors and impedance of said patient's body, based onmeasured values of said voltage drops; and determining the impedance ofsaid patient's body by using the measured value of the voltage drop dueto said patient's body and said obtained correlation formula.
 2. Anapparatus for measuring impedance of a patient's body comprising a pairof input electrodes and a pair of output electrodes adapted to contactwith spaced-apart portions of said patient's body; a plurality ofreference resistors each having a known resistance connected in serieswith input electrodes; means for supplying an electric current into acurrent path through said plurality of reference resistors, said inputelectrodes and said portions of the patient's body; voltage dropmeasuring means; switching means for momentarily and successivelyswitching over said voltage drop measuring means across any of saidreference resistors and/or across any groups of adjacent said referenceresistors, and across said pair of output electrodes to measure voltagedrops across the said reference resistors and/or voltage drops acrossthe groups of said reference resistors, and a voltage drop due to saidpatient's body when said current supplying means supplies an electriccurrent into said current path; and processing means for obtaining acorrelation formula between said resistances of said reference resistorsand impedance of patient's body on the basis of values of said voltagedrops measured by said voltage drop measuring means and determining theimpedance of said patient's body by using the measured value of thevoltage drop due to said patient's body and said obtained correlationformula.
 3. An apparatus for measuring impedance of a patient's bodycomprising: an insulating base upon which a patient stands by placingthe patient's feet on the insulating base; a first pair of conductiveelectrodes used as current supply terminals disposed on said base in aposition to be contacted by the underside of the patient's feet when thepatient is standing on the insulating base; a second pair of conductiveelectrodes used as voltage detecting terminals disposed on said base ina position to be contacted by the underside of the patient's feet whenthe patient is standing on the insulating base; said second pair ofconductive electrodes being set to 5 cm or more apart to said currentsupply terminal electrodes so as to eliminate the influence of apotential distribution; and a calculating device to calculate animpedance of a patient's body from current and voltage using said firstand second pairs of conductive electrodes.
 4. An apparatus for measuringimpedance of a patient's body comprising: an insulating base upon whicha patient stands by placing the patient's feet on the insulating base; afirst pair of conductive electrodes used as current supply terminalsdisposed on said base in a position to be contacted by the underside ofthe patient's toes when the patient is standing on the insulating base;a second pair of conductive electrodes used as voltage detectingterminals disposed on said base in a position to be contacted by theunderside of the patient's heels when the patient is standing on theinsulating base; said second pair of conductive electrodes being set to5 cm or more apart to said current supply terminals electrodes so as toeliminate the influence of a potential distribution; and a calculatingdevice to calculate an impedance of a patient's body from current andvoltage using said first and second pairs of conductive electrodes. 5.An apparatus for measuring bioelectrical impedance of a patient's bodycomprising an apparatus base of electrical insulating material, a firstconductive electrode provided on said base and adapted to contact a skinof said patient's body, a second conductive electrode provided on saidbase and adapted to contact a skin of said patient's body, an electriccurrent supplying device for supplying an electric current to said firstand second electrodes, a third conductive electrode provided on saidbase and adapted to contact a skin of said patient's body, a fourthconductive electrode provided on said base and adapted to contact a skinof said patient's body, and a calculating device for measuring a voltagebetween said third and fourth electrodes and calculating bioelectricalimpedance, whereby a distance through the skin of said patient's bodybetween said first or second electrode and said third or fourthelectrode is 5 cm or more when all of said first, second, third andfourth electrodes contact the skin of said patient's body.
 6. Anapparatus for measuring bioelectrical impedance of a patient's bodycomprising an apparatus base of electrical insulating material, a firstpair of conductive electrodes provided on said base and adapted tocontact a skin of said patient's body, an electric current supplyingdevice for supplying an electric current to said first pair ofelectrodes, a second pair of conductive electrodes provided on said baseand adapted to contact a skin of said patient's body, and a calculatingdevice for measuring a voltage between said second pair of electrodesand calculating bioelectrical impedance, whereby a distance through theskin of said patient's body between said first pair of electrodes andsaid second pair of electrodes is 5 cm or more when all of said firstpair of electrodes and said second pair of electrodes contact the skinof said patient's body.
 7. An apparatus for measuring impedance of apatient's body comprising: an insulating base upon which a patientstands by placing the patient's feet on the insulating base; a firstpair of elongated conductive electrodes used as current supply terminalsdisposed on said base in a position to be contacted by the underside ofthe patient's toes when the patient is standing on the insulating base,each of said pair of elongated conductive electrodes having alongitudinal axis extending in the fore and aft direction of thepatient's feet; a second pair of conductive electrodes used as voltagedetecting terminals disposed on said base in a position in alignmentwith said longitudinal axes to be contacted by the underside of thepatient's heels when the patient is standing on the insulating base; anda calculating device to calculate an impedance of a patient's body fromcurrent and voltage using said first and second pairs of conductiveelectrodes.
 8. An apparatus according to claim 7 wherein the first andsecond pair of conductive electrodes are raised slightly above theinsulating base so that they may be sensed by a patient standing on theinsulating base.
 9. An apparatus for measuring impedance of a patient'sbody comprising: an insulating base having an upper surface upon which apatient stands by placing the patient's feet on the insulating base; afirst pair of conductive electrodes used as current supply terminalsdisposed on said base in a position to be contacted by the underside ofthe patient's toes when the patient is standing on the insulating base;a second pair of conductive electrodes used as voltage detectingterminals disposed on said base in a position to be contacted by theunderside of the patient's heels when the patient is standing on theinsulating base; said first pair and said second pair of conductiveelectrodes being raised above the upper surface of the base by a slightamount in order to be sensed by the patient when standing on the base toinsure proper positioning of the patient's feet, and a calculatingdevice to calculate an impedance of a patient's body from current andvoltage using said first and second pairs of conductive electrodes.